Low power analog equalizer with current mode digital to analog converter

ABSTRACT

A low power analog equalizer is disclosed that provides up to twenty decibels (20 dB) of alternating current gain in a single stage of analog signal equalization. The analog equalizer comprises an operational amplifier coupled to two half circuits. Each half circuit comprises an impedance network capable of receiving an analog input voltage and generating a current signal that is inversely proportional to frequency, a current steering digital to analog converter capable of adjusting the gain of the operational amplifier, and a transistor and an amplifier coupled in a cascode configuration to create a low impedance node at the output of the impedance network. The analog equalizer is fabricated with 0.18 micron CMOS technology and operates at 1.8 volts.

RELATED APPLICATION

A related patent application by Abu-Hena M. Kamal and Jitendra Mohan has been filed concurrently with this patent application entitled “Low Power Analog Equalizer with Variable Op-Amp Gain” Ser. No. 10/001,767. The related patent application and the present patent application are commonly assigned to the assignee of the present patent application.

FIELD OF THE INVENTION

The present invention relates generally to the field of analog equalizers. More particularly, the present invention provides an improved analog equalizer that is capable of adaptively compensating for analog signal distortions that are caused by transmitting an analog signal through a cable to a receiver.

BACKGROUND OF THE INVENTION

A gigabit data transmission rate is equal to the transmission of one billion (10⁹) bits per second. The use of transceivers that are capable of gigabit data transmission rates is well known in digital communications technology. A gigabit Ethernet system enables data communication by transmitting and receiving data bits as analog signals over a Category 5 (CAT-5) unshielded twisted pair (UTP) cable. During the data communication process a receiver recovers the digital data from the transmitted analog signal. Data carrying signals are distorted severely as they are transmitted over a long cable length at a high frequency in the form of a multi level symbol alphabet, such as MLT-3 or PAM-17.

In such a case a gigabit receiver operates under several detrimental effects such as cable attenuation, echo from its own transmitter, etc. If channel distortion is not equalized or corrected, then the recovered clock signal from the received signal will be so erroneous that the recovered data will have an unacceptable bit error rate (BER), typically greater than 10⁻¹⁰. It is thus often necessary to improve the signal quality before attempting clock or data recovery. A cable equalizer circuit is implemented to compensate for the cable transmission loss and reshape the signal to its original (transmitted) waveform to improve inter-symbol interference (ISI) of the recovered data.

The transmission loss in decibels (dB) within the cable is linearly proportional to the cable length. The cable transmission loss in decibels (dB) is also proportional to the square root of the data transmission rate. The data transmission rate or transmission frequency and cable length are interdependent. An adaptive equalizer synthesizes a frequency response that is inversely proportional to that of the cable.

Various types of adaptive equalizers have been developed. Prior art adaptive equalizers improve signal quality either alone or in combination with digital finite impulse response (FIR) filters.

For example, U.S. Pat. No. 5,841,810 issued to Wong et al. on Nov. 24, 1998 describes a multistage adaptive equalizer. The input data signal in the Wong patent is successively filtered and magnitude weighted by successive adaptive filter circuits in accordance with corresponding adaptive control signals. In accordance with its respective adaptation control signal, each adaptive filter stage equalizes the data signal for a length of the cable through which the data signal was received. The adaptation control signal generator generates the individual adaptation control signals based upon an input equalization control signal. The input equalization control signal can be in the form of an analog voltage or in the form of a multiple bit digital signal. Each adaptive filter stage is configured with a constant gain signal path connected in parallel with a high frequency boosted, variable gain signal path. The input data signal is amplified in accordance with a first signal gain G1 that is constant over frequency F to provide one of the input signals to a signal summer. The input signal is also amplified in accordance with a second signal gain G2 that increases with frequency in a manner that is complementary to the complex cable loss characteristic of the cable. The resulting high frequency boosted signal is then amplified in accordance with a third signal gain G3. While constant over frequency F, the third signal gain G3 is a function of the corresponding adaptation control signal. The resulting high frequency boosted, variable gain signal is then summed with the constant gain signal to provide the partially equalized signal.

Another type of analog adaptive equalizer is described in U.S. Pat. No. 5,455,843 issued to Cherubini et al. on Oct. 3, 1995. Another type of analog adaptive equalizer is described in an article entitled “A 3.3 V Analog Adaptive Line-Equalizer For Fast Ethernet Data Communication” by J. N. Babanezhad in IEEE 1998 Custom Integrated Circuits Conference, pp. 343-346 (1998). Another type of device for equalizing channel distorted signals is described in U.S. Pat. No. 6,047,024 issued to How on Apr. 4, 2000.

It would be desirable to have an analog equalizer that is capable of operating at lower operating voltages than prior art analog equalizers.

It would also be desirable to have an analog equalizer that is capable of consuming less power than prior art analog equalizers.

It would also be desirable to have an analog equalizer that has a reduced die size compared to prior art analog equalizers.

SUMMARY OF THE INVENTION

The present invention is directed to an analog equalizer that is capable of adaptively compensating for analog signal distortions that are created during a gigabit data rate transmission of an analog signal through a cable to a receiver.

An advantageous embodiment of the present invention comprises an apparatus and method that adaptively compensates for gigabit channel impairment caused by the variation in channel length. A typical gigabit channel is a Category 5 (CAT-5) unshielded twisted pair (UTP) cable. The equalization created by the analog equalizer of the present invention provides a maximum boost of approximately twenty decibels (20 dB) which is sufficient to compensate for the attenuation experienced by a signal in a cable approximately one hundred twenty five meters (125 m) in length. The analog equalizer of the present invention synthesizes a frequency response that is inversely proportional to the frequency response of the cable. The analog equalizer of the present invention enables a one and eight tenths volt (1.8 volt) operation while significantly reducing power consumption and die size compared to prior art analog equalizers.

An advantageous embodiment of the present invention comprises a first low impedance node and a third low impedance node in a regulated cascode configuration, a current steering digital to analog converter (DAC), an operational amplifier with a feedback network, and an impedance network comprised of passive resistors and capacitors to create an impedance profile that is inversely proportional to frequency. The impedance network is AC coupled to the first low impedance node.

The high frequency portion of an input current that is provided to a first half circuit of the analog equalizer is steered from the first low impedance node through a first transistor and through the current steering DAC and summed at the third low impedance node with DAC steered currents from a second half circuit of the analog equalizer. The equivalent current that flows from the third low impedance node through a second transistor is then summed at a second low impedance node with an input current flowing through an input resistor and provided to the operational amplifier feedback network. The resulting current is then converted into voltage at the output of the analog equalizer using an RC filter network to bandlimit the signal. The analog signal is then converted to a digital signal with an analog to digital converter (ADC).

It is an object of the present invention to provide an apparatus and method for providing an analog equalizer that operates at a lower operating voltage than prior art analog equalizers.

It is another object of the present invention to provide an apparatus and method for providing an analog equalizer that consumes less power than prior art analog equalizers.

It is also an object of the present invention to provide an apparatus and method for providing an analog equalizer that has a smaller die size than prior art analog equalizers.

It is another object of the present invention to provide an apparatus and method for providing an analog equalizer that has greater linearity than prior art analog equalizers.

The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the Detailed Description of the Invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject matter of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.

Before undertaking the Detailed Description of the Invention, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: The terms “include” and “comprise” and derivatives thereof, mean inclusion without limitation, the term “or” is inclusive, meaning “and/or”; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, to bound to or with, have, have a property of, or the like; and the term “controller,” “processor,” or “apparatus” means any device, system or part thereof that controls at least one operation. Such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document. Those of ordinary skill should understand that in many instances (if not in most instances), such definitions apply to prior, as well as future uses of such defined words and phrases.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taking in conjunction with the accompanying drawings, wherein like numbers designate like objects, and in which:

FIG. 1 schematically illustrates a prior art data communication system comprising a transmitter and a receiver that is coupled to the transmitter by a cable that carries analog data signals;

FIG. 2 illustrates a graph of transmission loss values as a function of frequency and cable length for Category 5 unshielded twisted pair (UTP) cable;

FIG. 3 schematically illustrates a portion of a receiver that comprises an analog equalizer according to an advantageous embodiment of the present invention;

FIG. 4 schematically illustrates a more detailed circuit diagram of the analog equalizer of the present invention;

FIG. 5 schematically illustrates an impedance network adapted for use with the analog equalizer of the present invention;

FIG. 6 schematically illustrates a current mode digital to analog converter adapted for use with the analog equalizer of the present invention;

FIG. 7 illustrates a graph of the characteristics of an output signal as a function of frequency for a first index of attenuation and for a second index of attenuation; and

FIG. 8 illustrates a graph of an alternating current gain provided by the analog equalizer of the present invention as a function of frequency.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 through 8, discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged data communication system.

FIG. 1 schematically illustrates a prior art data communication system 100 comprising a transmitter unit 110 and a receiver unit 120. Cable 130 couples the transmitter unit 110 to the receiver unit 120. Transmitter 110 is capable of transmitting analog signals at gigabit data transmission rates over cable 130. The analog signals suffer attenuation as they pass through cable 130 to receiver unit 120. The amount of attenuation suffered by the analog signals depends upon several factors. Such factors include the frequency at which the analog signals are transmitted and the length of cable 130.

FIG. 2 illustrates a graph 200 of typical transmission loss values (dB) as a function of frequency (Hz) and cable length for Category 5 (CAT-5) unshielded twisted pair (UTP) cable. In FIG. 2 the transmission loss (TL) values in decibels (dB) are calculated as a function of frequency f in Hertz (Hz) and cable length (CL) in meters (m) as follows:

TL=CL*{[190*10⁻¹²*ƒ(Hz)]+[18*10⁻⁶*sqrt(ƒ(Hz))]}  (1)

where the letters “sqrt” represent the square root operation. The value of the transmission loss in decibels (dB) is linearly proportional to the cable length. The value of the transmission loss in decibels (dB) is also proportional to the square root of the data transmission rate.

FIG. 3 schematically illustrates a portion of receiver 120 that comprises analog equalizer 400 according to an advantageous embodiment of the present invention. Analog signals from cable 130 are received in automatic gain control (AGC) unit 300. Analog equalizer 400 is coupled to and receives analog signals from automatic gain control (AGC) unit 300. Analog equalizer 400 of the present invention provides equalization to the analog signals received from automatic gain control (AGC) unit 300. The equalized analog signals from analog equalizer 400 are then sent to analog to digital converter (ADC) 350. The digital signals from ADC 350 are then provided to other circuits (not shown) within receiver 120 for further processing.

FIG. 4 schematically illustrates an analog equalizer 400 of the present invention. Analog equalizer 400 is symmetrical with respect to operational amplifier (op-amp) 405. That is, analog equalizer 400 comprises a first half circuit 410 (on the left side of op-amp 405 in FIG. 2) and a second half circuit 415 (on the right side of op-amp 405 in FIG. 4). Half circuit 410 is a mirror image with respect to the half circuit 415, and vice versa. Analog equalizer 400 uses an operating voltage of one and eight tenths volts (1.8 volts) and is fabricated using 0.18 micron (μm) complementary metal oxide semiconductor (CMOS) technology.

Half circuit 410 receives voltage signal V_(IN) _(—) _(P) as an input from automatic gain control (AGC) unit 300. Half circuit 410 provides equalization to voltage signal V_(IN) _(—P) . The equalized output signal (V_(OP) _(—) _(EQ)) of voltage signal V_(IN) _(—) _(P) is an output from op-amp 405 on output signal line 420. Similarly, half circuit 415 receives voltage signal V_(IN) _(—) _(N) as an input from automatic gain control (AGC) unit 300. Half circuit 415 provides equalization to voltage signal V_(IN) _(—) _(N). The equalized output signal (V_(ON) _(—) _(EQ)) of voltage signal V_(IN) _(—) _(N) is an output from op-amp 405 on output signal line 460. A typical frequency value for voltage signal V_(IN) _(—) _(P) and for voltage signal V_(IN) _(—) _(N) is sixty two and one half megahertz (62.5 MHz).

The operation of half circuit 410 will be described first. As previously mentioned, voltage signal V_(IN) _(—) _(P) is obtained from the output of automatic gain control (AGC) unit 300. Voltage signal V_(IN) _(—) _(P) is provided simultaneously to Z1 network 425 and to a first terminal of input resistor R1. The output of Z1 network 425 is coupled to low impedance node SN1. The letters “SN” stand for “summing node.” A second terminal of input resistor R1 is coupled to low impedance node SN2. SN2 is coupled to a first input of op-amp 405. In one advantageous embodiment of the present invention, input resistor R1 may have a value of resistance equal to 24.0 kilohms.

As shown in FIG. 4, a feedback network comprising resistor R2 and capacitor C1 is coupled in parallel with op-amp 405. A first terminal of resistor R2 and a first terminal of capacitor C1 are each coupled to low impedance node SN2. A second terminal of resistor R2 and a second terminal of capacitor C1 are each coupled to output signal line 420. The feedback network comprising resistor R2 and capacitor C1 provides a low pass filter and a current to voltage converter for op-amp 405. In one advantageous embodiment of the present invention, the value of input resistor R1 and the value of resistor R2 are equal in order to maintain unity direct current (DC) gain. In one advantageous embodiment of the present invention, resistor R2 may have a value of resistance of 24.0 kilohms and capacitor C1 may have a capacitance value of 55.0 femtofarads. A femtofarad is 10⁻¹⁵ farad.

The output of Z1 network 425 is AC coupled to low impedance node SNl. Z1 network 425 converts the input voltage signal V_(IN) _(—) _(P) to a current signal I_(IN) _(—) _(P). The current signal I_(IN) _(—) _(P) that is output from Z1 network 425 is summed at low impedance node SN1 with a bias current I₁ from current source 430.

In one advantageous embodiment of the present invention, Z1 network 425 comprises a passive network 500 of resistors and capacitors as shown in FIG. 5. In one advantageous embodiment of passive network 500, the resistors may have the following values: R₅₁ is 29.0 kilohms, R₅₂ is 6.5 kilohms, R₅₃ is 6.5 kilohms, R₅₄ is 6.5 kilohms, R₅₅ is 4.0 kilohms, and R₅₆ is 500.0 ohms. In addition, the capacitors may have the following values: C₅₁ is 3.6 picofarads, C₅₂ is 1.2 picofarads, C₅₃ is 1.2 picofarads, C₅₄ is 1.2 picofarads, C₅₅ is 1.2 picofarads, and C₅₆ is 1.2 picofarads. A picofarad is 10⁻¹² farad. The effective impedance of passive network 500 at 62.5 MHz is approximately two and one half kilohms (2.5 kilohms).

Passive network 500 is only one example of Z1 network 425. The present invention is not limited to the specific Z1 network shown in FIG. 5. It is understood that other types of passive networks comprising resistors and capacitors may also be used.

As shown in FIG. 4, a first cascode configuration comprising amplifier 435 and transistor M1 creates low impedance node SN1 at the output of Z1 network 425. The source of transistor M1 is coupled to the input of amplifier 435. This arrangement creates low impedance node SN1 between transistor M1 and amplifier 435. The output of amplifier 435 is coupled to the gate of transistor M1.

A first input of digital to analog converter 440 (DAC 1) is coupled to low impedance node SN1. A second input of digital to analog converter 440 is coupled to the gate of transistor M1. A more detailed view of digital to analog converter 440 is shown in FIG. 6. Digital to analog converter 440 comprises eight (8) transistors, M_(DAC) 0 through M_(DAC) 7, that are coupled in a parallel configuration. That is, as shown in FIG. 6, the sources of each of the eight (8) transistors in digital to analog converter 440 are coupled to low impedance node SN1. The gates of each of the eight (8) transistors in digital to analog converter 440 are coupled to the gate of transistor M1.

Digital to analog converter 440 is a current steering DAC that is controlled by a three (3) bit digital word. That is, a three (3) bit digital word input to digital to analog converter 440 is used to generate eight (8) separate settings, from a setting of all zero bits (000) for the lowest level of current to a setting of all one bits (111) for the highest level of current. The setting provided to digital to analog converter 440 sets an index of attenuation alpha (α) that varies from a value of zero (0) representing zero attenuation to a value of one (1) representing maximum attenuation.

The combined bias current I₁ and the current I_(IN) _(—) _(P) are summed at low impedance node SN1 and then flow through transistor M1 and through digital to analog converter 440 (DAC 1). The high frequency portion of the input current is steered from low impedance node SN1 through transistor M1 and through digital to analog converter 440. Transistor M1 and digital to analog converter 440 have common gate to source voltage.

The drain of transistor M1 is coupled to low impedance node SN3. The voltage signal at the drain of transistor M1 is equal to V_(SB). As shown in FIG. 4 and in FIG. 6, the current signal at the drains of the eight (8) transistors, M_(DAC) 0 through M_(DAC) 7, of digital to analog converter 440 may be switched either to low impedance node SN3 or to low impedance node SN6 of half circuit 415.

The current through transistor M1 and through digital to analog converter 440 are summed at low impedance node SN3. The resulting current is also summed at low impedance node SN3 with a current from digital to analog converter 480 of half circuit 415.

Transistor M2 and amplifier 450 comprise a second cascode configuration that creates low impedance node SN3. The source of transistor M2 is coupled to the input of amplifier 450. This arrangement creates low impedance node SN3 between transistor M2 and amplifier 450. The output of amplifier 450 is coupled to the gate of transistor M2. The input of amplifier 450 and the source of transistor M2 are coupled to current source 455. Current source 455 generates a bias current equal to I₂.

The drain of transistor M2 is coupled to current source 445. Current source 445 provides bias current I₃ for the operation of transistor M2. The voltage signal at the drain of transistor M2 is equal to V_(OP). The current from transistor M1 and the current from digital to analog converter 440 (and the current from digital to analog converter 480) are summed at low impedance node SN3. The combined current at low impedance node SN3 is added together with the input current that is coming through input resistor R1 at low impedance node SN2 before going through the feedback network of op-amp 405. The resulting current is then converted into voltage at the output of analog equalizer 400 using an RC filter network to bandlimit the signal. The analog signal is then converted into a digital signal in analog to digital converter 350.

Input resistor R1 and resistor R2 (the op-amp feedback resistor) are set equal in order to maintain unity direct current (DC) gain. The overall gain A_(P) of half circuit 410 can be calculated as follows: $\begin{matrix} {A_{p} = {\frac{V_{OP\_ EQ}}{V_{IN\_ P}} = {{\frac{R2}{{Z1}(f)}\left( {1 - {2\alpha}} \right)} + \frac{R2}{R1}}}} & (2) \end{matrix}$

where Z1(f) is the impedance of Z1 network 425 as a function of frequency and where alpha (α) is an index of attenuation. Impedance Z1 is composed of resistors (R₅₁ through R₅₆) and capacitors (C₅₁ through C₅₆) Impedance Z1 decreases with frequency causing the input current I_(IN) _(—) _(P) to increase with frequency.

The input current I_(IN) _(—) _(P) does not experience any attenuation when alpha (α) equals zero (0). As the value of alpha (α) increases, the output of the system at a given frequency decreases. For short cable lengths, the input from automatic gain control (AGC) unit 300 is large and requires very little equalization. But for long cable lengths, the situation is reversed and much equalization is required. The input signal that receives a boost at high frequency by the high pass Z1 network 425 is attenuated in accordance with the strength of the input signal. The output signal is controlled by digital to analog converter 440 as shown in FIG. 7.

FIG. 7 illustrates how the output signal varies as a function of frequency for a first index of attenuation “alpha one” (α1) and for a second index of attenuation “alpha two” (α2). In the example shown in FIG. 7, the value of the index of attenuation α2 is greater than the value of the index of attenuation α1. Therefore, the values of the output signal for the index of attenuation α2 are lower than the values of the output signal for the index of attenuation α1. The peak value of each the two output signals is located at a frequency of approximately sixty two and one half megahertz (62.5 MHz).

At a frequency of 62.5 MHz the value of the impedance Z1 is two and four tenths kilohms (2.4 kilohms). This requires the value of the resistance of op-amp feedback resistor R2 to be twenty four kilohms (24.0 kilohms) for a twenty decibel (20 dB) boost if zero attenuation (α=0) is caused by digital to analog converter 440. For the same value of op-amp feedback resistor R2 the total attenuation (i.e., the no gain case) would require the value of alpha (α) to be set to forty five hundredths (0.45). The values of R1 and R2 are set equal so that the direct current (DC) gain is unity.

An expression for alpha (α) in terms of transistor conductances can be calculated as follows: $\begin{matrix} {\alpha = {\frac{i_{DAC}}{i_{IN}} = \frac{g_{DAC}}{g_{DAC} + g_{M1}}}} & (3) \end{matrix}$

where g_(DAC) is the effective conductance of digital to analog converter 440 and g_(M1) is the conductance of transistor M1.

The operation of half circuit 415 is similar to the operation of half circuit 410. Voltage signal V_(IN) _(—) _(N) is obtained from the output of automatic gain control (AGC) unit 300. Voltage signal V_(IN) _(—) _(N) is provided simultaneously to Z2 network 465 and to a first terminal of input resistor R3. The output of Z2 network 465 is coupled to low impedance node SN4. A second terminal of input resistor R3 is coupled to low impedance node SN5. SN5 is coupled to a second input of op-amp 405. In one advantageous embodiment of the present invention, input resistor R3 may have a value of resistance equal to 24.0 kilohms.

As shown in FIG. 4, a feedback network comprising resistor R4 and capacitor C2 is coupled in parallel with op-amp 405. A first terminal of resistor R4 and a first terminal of capacitor C2 are each coupled to low impedance node SN5. A second terminal of resistor R4 and a second terminal of capacitor C2 are each coupled to output signal line 460. The feedback network comprising resistor R4 and capacitor C2 provides a low pass filter and a current to voltage converter for op-amp 405. In one advantageous embodiment of the present invention, the value of input resistor R3 and the value of resistor R4 are equal in order to maintain unity direct current (DC) gain. In one advantageous embodiment of the present invention, resistor R4 may have a value of resistance of 24.0 kilohms and capacitor C2 may have a value of 55.0 femtofarads. A femtofarad is 10⁻¹⁵ farad.

The output of Z2 network 465 is AC coupled to low impedance node SN4. Z2 network 465 converts the input voltage signal V_(IN) _(—) _(N) to a current signal I_(IN) _(—) _(N). The current signal I_(IN) _(—) _(N) that is output from Z2 network 465 is summed at low impedance node SN4 with a bias current I₁ from current source 470.

In one advantageous embodiment of the present invention, Z2 network 465 comprises a passive network 500 of resistors and capacitors as shown in FIG. 5. Passive network 500 is only one example of Z2 network 465. The present invention is not limited to the specific Z2 network shown in FIG. 5. It is understood that other types of passive networks comprising resistors and capacitors may also be used.

As shown in FIG. 4, a third cascode configuration comprising amplifier 475 and transistor M3 creates low impedance node SN4 at the output of Z2 network 465. The source of transistor M3 is coupled to the input of amplifier 475. This arrangement creates low impedance node SN4 between transistor M3 and amplifier 475. The output of amplifier 475 is coupled to the gate of transistor M3.

A first input of digital to analog converter 480 (DAC 2) is coupled to low impedance node SN4. A second input of digital to analog converter 480 is coupled to the gate of transistor M3. Digital to analog converter 480 is identical in structure and function to digital to analog converter 440 shown in FIG. 6. Digital to analog converter 480 comprises eight (8) transistors, M_(DAC) 0 through M_(DAC) 7, that are coupled in a parallel configuration. That is, the sources of each of the eight (8) transistors in digital to analog converter 480 are coupled to low impedance node SN4. The gates of each of the eight (8) transistors in digital to analog converter 480 are coupled to the gate of transistor M3.

Digital to analog converter 480 is a current steering DAC that is controlled by a three (3) bit digital word. That is, a three (3) bit digital word input to digital to analog converter 480 is used to generate eight (8) separate settings, from a setting of all zero bits (000) for the lowest level of current to a setting of all one bits (111) for the highest level of current. The setting provided to digital to analog converter 480 sets an index of attenuation alpha (α) that varies from a value of zero (0) representing zero attenuation to a value of one (1) representing maximum attenuation.

The combined bias current I₁ and the current I_(IN) _(—) _(N) are summed at low impedance node SN4 and then flow through transistor M3 and through digital to analog converter 480 (DAC 2). The high frequency portion of the input current is steered from low impedance node SN4 through transistor M3 and through digital to analog converter 480. Transistor M3 and digital to analog converter 480 have common gate to source voltage.

The drain of transistor M3 is coupled to low impedance node SN6. The voltage signal at the drain of transistor M3 is equal to V_(SB). The current signal at the drains of the eight (8) transistors, M_(DAC) 0 through M_(DAC) 7, of digital to analog converter 480 may be switched either to low impedance node SN6 or to low impedance node SN3 of half circuit 410.

The current through transistor M3 and through digital to analog converter 480 are summed at low impedance node SN6. The resulting current is also summed at low impedance node SN6 with a current from digital to analog converter 440 of half circuit 410.

Transistor M4 and amplifier 490 comprises a fourth cascode configuration that creates low impedance node SN6. The source of transistor M4 is coupled to the input of amplifier 490. This arrangement creates low impedance node SN6 between transistor M4 and amplifier 490. The output of amplifier 490 is coupled to the gate of transistor M4. The input of amplifier 490 and the source of transistor M4 are coupled to current source 495. Current source 495 generates a bias current equal to I₂.

The drain of transistor M4 is coupled to current source 485. Current source 485 provides bias current I₃ for the operation of transistor M4. The voltage signal at the drain of transistor M4 is equal to V_(ON). The current from transistor M3 and the current from digital to analog converter 480 (and the current from digital to analog converter 440) are summed at low impedance node SN6. The combined current at low impedance node SN6 is added together with the input current that is coming through input resistor R3 at low impedance node SN5 before going through the feedback network of op-amp 405. The resulting current is then converted into voltage at the output of analog equalizer 400 using an RC filter network to bandlimit the signal. The analog signal is then converted into a digital signal in analog to digital converter 350.

Input resistor R3 and resistor R4 (the op-amp feedback resistor) are set equal in order to maintain unity direct current (DC) gain. The overall gain A_(N) of half circuit 415 can be calculated as follows: $\begin{matrix} {A_{N} = {\frac{V_{ON\_ EQ}}{V_{IN\_ N}} = {{\frac{R4}{{Z2}(f)}\left( {1 - {2\alpha}} \right)} + \frac{R4}{R3}}}} & (4) \end{matrix}$

where Z2(f) is the impedance of Z2 network 465 and where alpha (α) is an index of attenuation. Impedance Z2 is composed of resistors (R₅₁ through R₅₆) and capacitors (C₅₁ through C₅₆). Impedance Z2 decreases with frequency causing the input current I_(IN) _(—) _(N) to increase with frequency.

The input current I_(IN) _(—) _(N) does not experience any attenuation when alpha (α) equals zero (0). As the value of alpha (α) increases the output of the system at a given frequency decreases. For short cable lengths, the input from automatic gain control (AGC) unit 300 is large and requires very little equalization. But for long cable lengths, the situation is reversed and much equalization is required. The input signal that receives a boost at high frequency by the high pass Z2 network 465 is attenuated in accordance with the strength of the input signal. The output signal is controlled by digital to analog converter 480 as shown in FIG. 7.

At a frequency of sixty two and one half megahertz (62.5 MHZ) the value of the impedance Z2 is two and four tenths kilohms (2.4 kilohms). This requires the value of the resistance of op-amp feedback resistor R4 to be twenty four kilohms (24.0 kilohms) for a twenty decibel (20 dB) boost if zero attenuation (α=0) is caused by digital to analog converter 480. For the same value of op-amp feedback resistor R4 the total attenuation (i.e., the no gain case) would require the value of alpha (α) to be set to forty five hundredths (0.45). The values of R3 and R4 are set equal so that the direct current (DC) gain is unity.

In one advantageous embodiment of the present invention, analog equalizer 400 has seven (7) gain steps. In this embodiment, digital to analog converter 440 (and digital to analog converter 480) is capable of receiving three (3) input bits. The three (3) input bits set digital to analog converter 440 (and digital to analog converter 480) to eight (8) levels from level zero (0) to level seven (7) depending upon the strength of the analog input signals, V_(IN) _(—) _(P) and V_(IN) _(—) _(N).

When attenuation in the channel is at a maximum level, analog equalizer 400 must provide a maximum level of equalization. The maximum level of equalization corresponds to the largest setting of the DAC unit. The largest setting of the DAC unit is level seven (7) corresponding to all three (3) DAC input bits set equal to one (1). The smallest setting of the DAC unit is level zero (0) corresponding to all three (3) DAC input bits set equal to zero (0). Analog equalizer 400 produces no equalization gain when the DAC unit is set at level zero (0). Analog equalizer 400 produces maximum equalization gain with the DAC unit is set at level seven (7).

The alternating current (AC) gain provided by analog equalizer 400 (as a function of the DAC settings) at a frequency of 62.5 MHz for a slow process, at a temperature of one hundred fifty degrees Centigrade (150° C.), with a power supply voltage equal to 1.6 volts is given below in the following table.

DAC Setting Gain (dB) 0 4.28 1 8.85 2 11.90 3 14.15 4 15.96 5 17.45 6 18.73 7 19.84

The alternating current (AC) gain provided by analog equalizer 400 (as a function of analog input signal frequency) at three different frequencies for a slow process, at a temperature of one hundred fifty degree Centigrade (150° C.), with power supply voltage equal 1.6 volts is given below in the following table.

Analog Gain at Gain at signal minimum maximum input DAC DAC frequency Setting setting (MHz) (dB) (dB) 1.0 0.403 4.43 30.0 3.86 17.85 62.5 4.28 19.84

FIG. 8 illustrates a graph of alternating current (AC) gain response provided by the analog equalizer 400 as a function a frequency. The designation “EQ_ctrl” in FIG. 8 refers to a DAC setting. For example, EQ_ctrl=7 signifies the maximum DAC setting (three bits equal to 111) and EQ_ctrl=0 signifies the minimum DAC setting (three bits equal to 000). The maximum amount of AC gain is approximately twenty decibels (20 dB).

Analog equalizer 400 of the present invention provides a significantly improved apparatus and method of analog signal equalization compared to the prior art. For example, analog equalizer 400 uses an operating voltage of one and eight tenths volts (1.8 volts). The analog equalizer described in U.S. Pat. No. 5,841,810 issued to Wong et al. on Nov. 24, 1998 (the “Wong patent”) uses an operating voltage of three and three tenths volts (3.3 volts). A 3.3 volt analog equalizer is also described in a prior art article entitled “A 3.3V Analog Adaptive Line-Equalizer For Fast Ethernet Data Communication” by J. N. Babanezhad in IEEE 1998 Custom Integrated Circuits Conference, pp. 343-346 (1998). Analog equalizer 400 of the present invention provides equalization of analog signals using a lower operating voltage than that of prior art analog equalizers.

Analog equalizer 400 of the present invention also consumes less power than prior art analog equalizers. Analog equalizer 400 uses less than three milliamps (3 mA) of current at 1.8 volts. The analog equalizer of the Wong patent uses twenty four milliamps (24 mA) of current at 3.3 volts. Analog equalizer 400 of the present invention provides equalization of analog signals while consuming less power than prior art analog equalizers.

Analog equalizer 400 of the present invention also has a smaller die size than prior art analog equalizers. Analog equalizer 400 uses less than one fourth of the die size used by the analog equalizer of the Wong patent.

Analog equalizer 400 of the present invention also provides greater linearity compared to prior art analog equalizers. Analog equalizer 400 provides more than sixty decibels (60 dB) of linearity compared to less than forty decibels (40 dB) of linearity provided by the analog equalizer of the Wong patent.

In addition, the Wong patent provides equalization to analog signals by using a plurality of boost stages. Each boost stage provides a portion of the total gain needed to compensate for the total cable transmission loss. In contrast, analog equalizer 400 of the present invention comprises a single stage equalizer that is capable of generating up to twenty decibels (20 dB) of gain.

The above examples and description have been provided only for the purpose of illustration, and are not intended to limit the invention in any way. As will be appreciated by the skilled person, the invention can be carried out in a great variety of ways, employing more than one technique from those described above, all without exceeding the scope of the invention. 

What is claimed is:
 1. An analog equalizer comprising: an operational amplifier; a first half circuit coupled to a first input of said operational amplifier, said first half circuit capable of providing alternating current gain to a first input voltage signal provided to a first input of said analog equalizer, said first half circuit comprising: a first impedance network having an input coupled to said first input of said analog equalizer and having an output coupled to a first low impedance node, said first impedance network having an impedance Z1 that generates an output current signal that is inversely proportional to frequency; a first input resistor R1 having an input coupled to said first input of said analog equalizer and having an output coupled to a second low impedance node coupled to said first input of said operational amplifier; a first transistor and a first amplifier coupled together in a first cascode configuration to create said first low impedance node at an output of said first impedance network, wherein an output from said first transistor is coupled to a third low impedance node; a first current steering digital to analog converter having a first input coupled to said first low impedance node and a second input coupled to a gate of said first transistor, wherein a first output of said first current steering digital to analog converter is coupled to said third low impedance node; a second transistor and a second amplifier coupled together in a second cascode configuration to create said third low impedance node at said output of said first transistor and at said first output of said first current steering digital to analog converter, wherein an output of said second transistor is coupled to said second low impedance node; and a first feedback network coupled to said second low impedance node and to a first output signal line of said operational amplifier.
 2. An analog equalizer as claimed in claim 1 wherein a second output of said first current steering digital to analog converter is coupled to a sixth low impedance node of a second half circuit of said analog equalizer.
 3. An analog equalizer as claimed in claim 1 wherein said first feedback network comprises a resistor R2 and a capacitor C1 coupled in parallel to provide a low pass filter and a current to voltage converter for said first output signal line of said operational amplifier.
 4. An analog equalizer as claimed in claim 3 wherein a value of resistance of input resistor R1 and a value of resistance of resistor R2 are equal to maintain unity direct current gain.
 5. An analog equalizer as claimed in claim 3 wherein an overall gain A_(p) of said first half circuit is equal to: $A_{p} = {{\frac{R2}{{Z1}(f)}\left( {1 - {2\alpha}} \right)} + \frac{R2}{R1}}$

where Z1(f) is an impedance of said first impedance network as a function of frequency, and R1 is a value of resistance of said input resistor R1, and R2 is a value of resistance of said second resistor R2, and alpha (α) is an index of attenuation.
 6. An analog equalizer as claimed in claim 1 wherein said first current steering digital to analog converter is capable of being set to a value of current within a range of values of current, wherein each value of current in said range of values of current provides a level of alternating current gain within said first half circuit of said analog equalizer.
 7. An analog equalizer as claimed in claim 6 wherein said first current steering digital to analog converter is capable of receiving a three input bit digital signal capable of setting eight levels of current through said first current steering digital to analog converter.
 8. An analog equalizer as claimed in claim 1 capable of operating at an operating voltage of approximately one and eight tenths volts (1.8 volts).
 9. An analog equalizer as claimed in claim 1 fabricated using 0.18 micron complementary metal oxide semiconductor technology.
 10. An analog equalizer as claimed in claim 1 wherein said analog equalizer comprises a single stage equalizer capable of providing up to twenty decibels (20 dB) of alternating current gain.
 11. An analog equalizer as claimed in claim 1 wherein said first input of said analog equalizer is coupled to an output of an automatic gain control circuit and receives said first input voltage signal from said automatic gain control circuit.
 12. An analog equalizer as claimed in claim 1 wherein said first input voltage signal has a maximum frequency value of approximately sixty-two and one half megaHertz (62.5 MHz).
 13. An analog equalizer as claimed in claim 1 wherein said first impedance network comprises a passive network of resistors and capacitors.
 14. An analog equalizer as claimed in claim 1 further comprising: a second half circuit coupled to a second input of said operational amplifier, said second half circuit capable of providing alternating current gain to a second input voltage signal provided to a second input of said analog equalizer, said second half circuit comprising: a second impedance network having an input coupled to said second input of said analog equalizer and having an output coupled to a fourth low impedance node, said second impedance network having an impedance Z2 that generates an output current signal that is inversely proportional to frequency; a second input resistor R3 having an input coupled to said second input of said analog equalizer and having an output coupled to a fifth low impedance node coupled to said second input of said operational amplifier; a third transistor and a third amplifier coupled together in a third cascode configuration to create said fourth low impedance node at an output of said second impedance network, wherein an output from said third transistor is coupled to a sixth low impedance node; a second current steering digital to analog converter having a first input coupled to said fourth low impedance node and a second input coupled to a gate of said third transistor, wherein a first output of said second current steering digital to analog converter is coupled to said sixth low impedance node; a fourth transistor and a fourth amplifier coupled together in a fourth cascode configuration to create said sixth low impedance node at said output of said third transistor and at said first output of said second current steering digital to analog converter, wherein an output of said fourth transistor is coupled to said fifth low impedance node; and a second feedback network coupled to said fifth low impedance node and to a second output signal line of said operational amplifier.
 15. An analog equalizer as claimed in claim 14 wherein a second output of said second current steering digital to analog converter is coupled to said third low impedance node of said first half circuit of said analog equalizer.
 16. An analog equalizer as claimed in claim 14 wherein said second feedback network comprises a resistor R4 and a capacitor C2 coupled in parallel to provide a low pass filter and a current to voltage converter for said second output signal line of said operational amplifier.
 17. An analog equalizer as claimed in claim 16 wherein a value of resistance of input resistor R3 and a value of resistance of resistor R4 are equal to maintain unity direct current gain.
 18. An analog equalizer as claimed in claim 16 wherein an overall gain A_(N) of said second half circuit is equal to: $A_{N} = {{\frac{R4}{{Z2}(f)}\left( {1 - {2\alpha}} \right)} + \frac{R4}{R3}}$

where Z2(f) is an impedance of said second impedance network as a function of frequency, and R3 is a value of resistance of said input resistor R3, and R4 is a value of resistance of said resistor R4, and alpha (α) is an index of attenuation.
 19. An analog equalizer as claimed in claim 14 wherein said second current steering digital to analog converter is capable of being set to a value of current within a range of values of current, wherein each value of current in said range of values of current provides a level of alternating current gain within said second half circuit of said analog equalizer.
 20. An analog equalizer as claimed in claim 19 wherein said second current steering digital to analog converter is capable of receiving a three input bit digital signal capable of setting eight levels of current through said second current steering digital to analog converter.
 21. An analog equalizer as claimed in claim 14 capable of operating at an operating voltage of approximately one and eight tenths volts (1.8 volts).
 22. An analog equalizer as claimed in claim 14 fabricated using 0.18 micron complementary metal oxide semiconductor technology.
 23. An analog equalizer as claimed in claim 14 wherein said analog equalizer comprises a single stage equalizer capable of providing up to twenty decibels (20 dB) of alternating current gain.
 24. An analog equalizer as claimed in claim 14 wherein said second input of said analog equalizer is coupled to an output of an automatic gain control circuit and receives said second input voltage signal from said automatic gain control circuit.
 25. An analog equalizer as claimed in claim 14 wherein said second input voltage signal has a maximum frequency value of approximately sixty-two and one half megaHertz (62.5 MHz).
 26. An analog equalizer as claimed in claim 14 wherein said second impedance network comprises a passive network of resistors and capacitors.
 27. A method for equalizing an analog input voltage signal, said method comprising the steps of: providing a first analog input voltage signal to a first input of an analog equalizer; providing said first analog input voltage signal to a first impedance network of said analog equalizer, said first impedance network having an output coupled to a first low impedance node; providing said first analog input voltage signal to an input resistor R1 of said analog equalizer, said input resistor R1 having an output coupled to a second low impedance node coupled to a first input of an operational amplifier of said analog equalizer; generating a current signal at an output of said first impedance network that is inversely proportional to frequency; providing said current signal from said first impedance network to a first transistor and a first amplifier coupled together in a first cascode configuration to create said first low impedance node; providing said current signal from said first impedance network to a first current steering digital to analog converter having a first input coupled to said first low impedance node and having a second input coupled to a gate of said first transistor; providing an output current signal from an output of said first transistor to a third low impedance node created by a second transistor and a second amplifier coupled together in a second cascode configuration; providing an output current signal from a first output of said first current steering digital to analog converter to said third low impedance node; summing at said third low impedance node said output current signal from said first transistor, and said output current signal from said first output of said first current steering digital to analog converter; providing to said first input of said operational amplifier a current signal at said second low impedance node that combines said output current signal from said third low impedance node and a current signal from said input resistor R1; and setting a value of alternating current gain for said current signal provided to said first input of said operational amplifier by setting a value of current through said first current steering digital to analog converter.
 28. A method as claimed in claim 27 further comprising the steps of: providing to said third low impedance node a current signal from an output of a second current steering digital to analog converter of a second half circuit of said analog equalizer; summing at said third low impedance node said output current signal from said first transistor, and said output current signal from said first output of said first current steering digital to analog converter, and said output current signal from said output of said second current steering digital to analog converter; and providing to said first input of said operational amplifier a current signal at said second low impedance node that combines said output current signal from said third low impedance node and a current signal from said input resistor R1; and setting a value of alternating current gain for said current signal provided to said first input of said operational amplifier by setting a value of current through said first current steering digital to analog converter and by setting a value of current through said second current steering digital to analog converter.
 29. A method as claimed in claim 28 further comprising the step of: setting a value of resistance of said input resistor R1 equal to a value of resistance of said resistor R2 to maintain unity direct current gain.
 30. A method as claimed in claim 28 wherein a value of gain A_(P) provided to said first input voltage signal is equal to: $A_{p} = {{\frac{R2}{{Z1}(f)}\left( {1 - {2\alpha}} \right)} + \frac{R2}{R1}}$

where Z1(f) is an impedance of said first impedance network as a function of frequency, and R1 is a value of resistance of said input resistor R1, and R2 is a value of resistance of said resistor R2, alpha (α) is an index of attenuation.
 31. A method as claimed in claim 27 wherein said step of setting a value of alternating current gain for said current signal provided to said first input of said operational amplifier by setting a value of current through said first current steering digital to analog converter comprises the step of: providing a digital input signal to said digital to analog converter to select a level of current through said first current steering digital to analog converter.
 32. A method as claimed in claim 27 further comprising the steps of: fabricating said analog equalizer with 0.18 micron complementary metal oxide semiconductor technology; and operating said analog equalizer at an operating voltage of approximately one and eight tenths volts (1.8 volts).
 33. A method as claimed in claim 27 further comprising the step of: providing up to twenty decibels (20 dB) of alternating current gain to said first input voltage signal in a single stage of analog signal equalization.
 34. A method for equalizing an analog input voltage signal as claimed in claim 27, said method further comprising the steps of: providing a second analog input voltage signal to a second input of said analog equalizer; providing said second analog input voltage signal to a second impedance network of said analog equalizer, said second impedance network having an output coupled to a fourth low impedance node; providing said second analog input voltage signal to an input resistor R3 of said analog equalizer, said input resistor R3 having an output coupled to a fifth low impedance node coupled to a second input of said operational amplifier of said analog equalizer; generating a current signal at an output of said second impedance network that is inversely proportional to frequency; providing said current signal from said second impedance network to a third transistor and a third amplifier coupled together in a third cascode configuration to create said fourth low impedance node; providing said current signal from said second impedance network to a second current steering digital to analog converter having a first input coupled to said fourth low impedance node and having a second input coupled to a gate of said third transistor; providing an output current signal from an output of said third transistor to a sixth low impedance node created by a fourth transistor and a fourth amplifier coupled together in a fourth cascode configuration; providing an output current signal from a first output of said second current steering digital to analog converter to said sixth low impedance node; summing at said sixth low impedance node said output current signal from said third transistor, and said output current signal from said first output of said second current steering digital to analog converter; providing to said second input of said operational amplifier a current signal at said fifth low impedance node that combines said output current signal from said sixth low impedance node and a current signal from said input resistor R3; and setting a value of alternating current gain for said current signal provided to said second input of said operational amplifier by setting a value of current through said second current steering digital to analog converter.
 35. A method as claimed in claim 34 further comprising the steps of: providing to said sixth low impedance node a current signal from an output of said first current steering digital to analog converter of said first half circuit of said analog equalizer; summing at said sixth low impedance node said output current signal from said third transistor, and said output current signal from said first output of said second current steering digital to analog converter, and said output current signal from said output of said first current steering digital to analog converter; providing to said second input of said operational amplifier a current signal at said fifth low impedance node that combines said output current signal from said sixth low impedance node and a current signal from said input resistor R3; and setting a value of alternating current gain for said current signal provided to said second input of said operational amplifier by setting a value of current through said second current steering digital to analog converter and by setting a value of current through said first current steering digital to analog converter.
 36. A method as claimed in claim 34 further comprising the step of: setting a value of resistance of said input resistor R3 equal to a value of resistance of said resistor R4 to maintain unity direct current gain.
 37. A method as claimed in claim 34 wherein a value of gain A_(N) provided to said second input voltage signal is equal to: $A_{p} = {{\frac{R2}{{Z1}(f)}\left( {1 - {2\alpha}} \right)} + \frac{R2}{R1}}$

where Z2(f) is an impedance of said second impedance network as a function of frequency, and R3 is a value of resistance of said input resistor R3, and R4 is a value of resistance of said resistor R4, and alpha (α) is an index of attenuation.
 38. A method as claimed in claim 34 wherein said step of setting a value of alternating current gain for said current signal provided to said second input of said operational amplifier by setting a value of current through said second current steering digital to analog converter comprises the step of: providing a digital input signal to said first current steering digital to analog converter to select a level of current through said first current steering digital to analog converter.
 39. A method as claimed in claim 34 further comprising the steps of: fabricating said analog equalizer with 0.18 micron complementary metal oxide semiconductor technology; and operating said analog equalizer at an operating voltage of approximately one and eight tenths volts (1.8 volts).
 40. A method as claimed in claim 34 further comprising the step of: providing up to twenty decibels (20 dB) of alternating current gain to said second input voltage signal in a single stage of analog signal equalization. 